Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and at the time of engine high load operation, and increasing the mechanical compression ratio and delaying the closing timing of the intake valve as the engine load becomes lower in a state holding an actual compression ratio constant at the times of these engine medium and high load operations (for example, see Japanese Patent Publication (A) No. 2004-218522).
In this regard, a spark ignition type internal combustion engine improving a heat efficiency at the time of vehicle operation to obtain a better fuel consumption efficiency by making the mechanical compression ratio maximum so as to obtain a maximum expansion ratio at the time of engine low load operation and by making the actual compression ratio at the time of engine low load operation substantially the same actual compression ratio as the time of engine medium and high load operation, has been proposed by the present applicant. In general, in a spark ignition type internal combustion engine, the higher the expansion ratio, the longer the time during which a pushdown force acts on the piston at the time of the expansion stroke. As a result, the heat efficiency rises. In the spark ignition type internal combustion engine proposed by the applicant, the expansion ratio is made maximum at the time of engine low load operation, so it is possible to obtain a high heat efficiency at the time of engine low load operation.
On the other hand, at the time of engine cold start, the temperature of an exhaust purification catalyst provided at the internal combustion engine (for example, a three-way catalyst) is lower than the activation temperature. For this reason, at the time of cold startup, it is necessary to rapidly raise the temperature of the exhaust purification catalyst. In this regard, as explained above, the larger the expansion ratio, the longer the time that a pushdown force acts on the piston at the time of the expansion stroke. This means that the larger the expansion ratio, the lower the temperature of the exhaust gas exhausted from the engine body. Therefore, if raising the expansion ratio at the time of cold startup, it is no longer possible to rapidly raise the temperature of the exhaust purification catalyst.
Further, at the time of engine cold start, the purification rate of unburnt HC by the exhaust purification catalyst falls. For this reason, at the time of engine cold start, it is necessary to reduce the HC contained in the exhaust gas exhausted from the engine body as much as possible. In this regard, if raising the actual compression ratio or raising the expansion ratio, the unburnt HC in the exhaust gas exhausted from the engine body tends to increase.